Corrosion resistant austenitic steel

ABSTRACT

A substantially non-porous, austenitic stainless steel that is resistant to dilute sulfuric acid and to chloride pitting, and a method of making it, are disclosed. The steel includes from 21-45 percent manganese, from 10-30 percent chromium, from 1-5 percent molybdenum, from 0.85-3 percent nitrogen, up to 2 percent silicon, up to 1 percent carbon and the balance iron and residuals. In addition to containing elements within the abovenoted composition ranges, the alloys must be such that: 1. %Cr+ %Mo + 0.8(%Mn) - 11.8(%N - 0.1) &gt; OR = 28.5 2. 30(%C + %N) + 0.5 (%Mn)/%Cr + %Mo + 1.5 (%Si) &gt; OR = 1.5 The steels are cast and worked to avoid a dwell period in the temperature range 1,000*-1,600*F.

14 1 Nov. 12, 1974 CORROSION RESISTANT AUSTENITIC STEEL [75] Inventor: Albert G. Hartline, III;

Brackenridge, Pa.

[73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

122 Filed: Oct. 4, 1973 21 Appl. No.: 403,347

' Primary Examiner-Hyland Bizot Attorney, Agent, or Firm-Vincent G. Gioia; Robert F. Dropkin [57] ABSTRACT A substantially non-porous, austenitic stainless steel that is resistant to dilute sulfuric acid and t0 chloride pitting, and a method of making it, are disclosed. The steel includes from 21-45 percent manganese, from -30 percent chromium, from l-5 percent molybdenum, from 0.85-3 percent nitrogen, up to 2 percent [52 U.S. c1. .[75/122, 75/126 B, 75/126 J, Silicon, up w 1 Percent Carbon and the balance iron 75/129 and residuals. In addition to containing elements 51 Int. Cl. C22C 39/14, C22C 33/00 Within the above-noted cflmpositioh ranges, lhe u"0yS 58 Field of Search 75/122, 126 B, 129 e hes that 1. %Cr %Mo 0.8(%Mn) 11.8(%N 0.1( g [56] References Cited (%Si) 2 1.5 j 132 g f 75/126 B The steels are cast and worked to avoid a dwell period J 75/126 B in the temperature range l,O00-l,600F. 8 Claims, 1 Drawing Figure Austen/re -Ferrire ,4

a g 2 Two Phase 40 a Fe 0 I0 20 '50 40 Manganese 1 CORROSION RESISTANT AUSTENITIC STEEL BACKGROUND OF THE INVENTION Corrosion resistant steels, known as stainless steels, have long been known and are presently available with a variety of properties. Austenitic stainless steels, which are those consisting substantially of a single austenite phase, possess the best properties of corrosion resistance and good mechanical properties, particularly at high temperature. Austenitic stainless steels in the past have been steels in which chromium and nickel are the principal alloying agents. However, nickel is not an abundant metal and the increased demand for it has increased its price and made its supply uncertain, particularly in critical times. Substitutes for nickel in the chromium-nickel austenitic stainless steels have long been sought. Recently the combined use of manganese, nitrogen and chromium in carefully balanced amounts has produced an austenitic stainless steel. This steel is described in US. Pat. application Ser. No. 251,637, filed May'8, 1972; and although it is an excellent steel, it is somewhat subject to attack by sulfuric acid and chloride environments.

SUMMARY OF THE INVENTION The present invention is a chromium-manganesemolybdenum-nitrogen steel that is substantially nonporous, austenitic and highly resistant to attack by sulfuric acid and chloride environments. The alloy of this invention contains from 21-45 percent manganese, from -30 percent chromium, from 1-5 percent molybdenum, from 0.85-3 percent nitrogen, up to 2 percent silicon, up to 1 percent carbon and the balance iron and residuals. All compositions in this specification and the following claims are in percent by weight of the total composition unless otherwise specified.

In addition to the foregoing ranges, the composition of the alloys must be balanced in accordance with the following equations:

1. %Cr %Mo O.8(%Mn) 11.8(%N 0.1) g

28.5 2. 30(%C %N) 0.5(%Mn)/%Cr+ %Mo-l- 1.5

In the alloys of the present invention, chromium must be present to produce the same effect that it does in prior art alloys. The alloys of this invention must contain from 10-30 percent chromium. At least 10 percent 2 is required to give the steel its outstanding corrosion resistance. Chromium also has a secondary effect upon the strength of the steel and is a primary element in increasing the steels solubility for nitrogen. An upper limit of 30 percent chromium is imposed as chromium is a ferrite former and excessive amounts of ferrite might form with higher chromium levels and in turn degrade the properties of the steel. A preferred chromium content is in the range of 15-27 percent in that steels containing this range of chromium are easy to process while still having good corrosion resistance and strength.

The manganese in the alloy of this invention is present in amounts of from 2145 percent. Since manganese is an austenitizer and increases the solubility of nitrogen in the steel, amounts in excess of 21 percent are required. An upper limit of 45 percent, and preferably an upper limit of 30 percent, manganese is imposed for economic considerations and because manganese tends to attack furnace refractories.

Nitrogen, a strong austenitizer, should be present in the steel in amounts of from 0.85-3 percent. At least 0.85 percent is required for its austenitizing effect and because it is the primary strengthening element of the steel. Amounts of nitrogen in excess of 3 percent tend to yield-porous ingots which are not satisfactory. The nitrogen content of the alloy of this invention preferably is from l.05-l.5 percent.

The molybdenum content of the alloy of this invention must be between l-5 percent. Although molybdenum has been known as an alloy additive in the past, in the present invention the molybdenum may be employed to reduce the amount of chromium and manganese employed, may be employed to increase the solubility of nitrogen, and most importantly reduces the susceptibility of the resultant alloy to attack by dilute sulfuric acid and to pitting attack by chlorides. The molybdenumcontaining alloy of this invention is as strong or stronger than the above referred to chromium-manganese-nitrogen alloys.

Carbon, of course, is a well-known austenitizer and strengthener for steels and is employed in the alloys of this invention in amounts up to 1 percent. The concentration of carbon must be maintained below that level in that larger amounts can remove chromium from solid solution by combining with it to form chromium carbides and because carbon can reduce the solubility of the steel for nitrogen by occupying interstitial sites normally filled with nitrogen. It is preferred that less than 0.15 percent carbon be present in the alloy of this invention. Higher carbon contents require higher annealing temperatures to put carbides into solution.

The alloys of the present invention may tolerate silicon concentrations as high as 2 percent but preferably the silicon is below 1 percent. Higher quantities of silicon tend to remove manganese from the alloy in the form of manganese silicates and tend to form inclusions in the steel.

Although the residuals in the iron need not be identified and do not significantly affect the properties of the alloy, the usual residuals may be identified as phosphorus, sulfur, tungsten, cobalt and nickel.

Since the stainless steel composition of this invention is desirably a substantially one-phase austenitic mate rial, thermal treatments that tend to precipitate other phases should be avoided. Although the alloys of this invention are not particularly sensitive to precipitation of other phases, the method of preparation employed should avoid long dwell periods in the l,000-l,600F temperature range. Long dwell periods would be characterized by furnace cooling. For ordinary thicknesses air cooling or quenching are sufficient to carry the alloy through the l,000-1,600F range quickly enough to avoid precipitation of detrimental phases such as sigma phase.

DETAILED DESCRIPTION OF THE INVENTION The accompanying drawing illustrates two plots of the 1.0 percent nitrogen section of'the iron-chromiummanganese-nitrogen quaternary phase diagram. One plot illustrates that section wherein the alloy contains only residual amounts of molybdenum while the molybdenum. plot illustrates that section containing 3 percent molybedenum.

As indicated in equations (1) and (2) set forth above, molybdenum has a significant effect both on the austenitic structure of the alloy and on the ability of the alloy to maintain nitrogen in solution both in the liquid phase and in the resultant solid phase. Molybdenum within the composition limits set forth herein appears to be a replacement for chromium in that the combined molybdenum-chromium composition of the alloys determines where a two-phase austenite-ferrite alloy structure begins as compared with a single phase austenite structure, as well as where nitrogen precipitates from solution causing ingot porosity. In the drawing the area above line 1-A and 1'A generally represents compositions where a two-phase alloy of austenite and ferrite exists. As mentioned above, this two-phase system is undesirable because it does not have the good mechanical or chemical properties of a single-phase austenitic alloy. The area below the lines l-A and 1A are single-phase austenitic alloys.

Equation (2) defines the lines 2B and 2'B. The area below these lines represents compositions where nitrogen comes out of solution during solidification and creates porous ingots. The areas above the lines 2B and 2'B' is where nitrogen remains in solution during solidification and non-porous ingots are formed.

The areas A-C-Band AC'-B therefor represent the areas in which the alloys of this invention and of the prior art fall for this particular cross section of the quaternary phase diagrams illustrated. The alloys of the present invention fall in the area ACB. It may be noted that a single-phase alloy having no porousity may be obtained with substantially less chromium and with substantially less manganese in accordance with the present invention.

To demonstrate the benefits of this invention, four alloys were prepared having compositions set forth in Table I.

TABLE I Element Weight Allov 1 Alloy 2 Alloy 3 Allov 4 Mn 30.40 29.98 30.39 30.39 Cr 20.45 20.34 19.97 20.02 Mo 0.033 1.02 1.96 2.95 N 1.00 1.18 1.18 1.18 Ni 0.26 0.23 0.24 0.24 Cu 0.20 0.16 0.16 0.16 Si 0.48 0.40 0.36 0.42 C 0.092 0.062 0.062 0.064 5 0.009 0.010 0.011 0 011 P 0.014 0.008 0.008

The alloys of Table l are substantially the same alloys with the exception of the molybdenum content. In effect, the alloys are all nominally 30 percent manganese, 20 percent chromium, 1 percent nitrogen alloys containing respectively a residual amount, 1, 2 and 3 percent molybdenum.

The alloys described in Table I were all prepared in the same way. All alloys were prepared from melted materials in an air induction furnace and were composited of commercial grades of ferroalloys and pure elements. The heats were cast from approximately 2,650F into 35 pound cast iron ingot molds. After solidification, the ingots were examined for porosity which was not observed in any of the alloys. The hot processing of the alloys consisted of grinding to remove casting imperfections, heating the ingots at 2,250F for an appropriate amount of time, and hot rolling to the desired width and thickness. While the hot strength ap peared to increase with the addition of molybdenum,

the ingots were rolled without incident. The hot rolled materials were annealed at 1,950F on a schedule of 120 minutes per inch of material thickness, subsequently blasted and pickled in a mixture of 15 percent nitric acid and 3 percent hydrofluoric acid, cold rolled percent to further homogenize the structure, final annealed at 1,950F on the same schedule as mentioned above and again pickled. At various stages of the processing, samples were obtained to determine the mechanical properties of the metals. These properties are set forth in Table II. The strength data were obtained after annealing for 7 minutes at 1,950F because this treatment produces a condition of minimum strength and maximum ductility in all alloys so that truly comparative data are obtained. It may be noted that the addition of molybdenum does not have a significant effect upon the mechanical properties of a nominal 20 percent chromium-30 percent manganese-1 percent nitrogen stainless steel.

The alloys of this invention are resistant to pitting attack in chloride evironments. The resistance to pitting attack was measured in a crevice corrosion test wherein standardized specimens of the various alloys are immersed in 10% ferric chloride solution for 72 hours. The pitting attack is measured by the mean weight loss of a number ofsamples of each alloy. The

TABLE II Tensile Strength Allov Number As Annealed 1 3 4 0.2% Yield Strength (KS1) I Longitudinal 97.5 105.9- 108.4- 108.9

99.2 109.1 109.6 109.3. Transverse 98.2 105.8- 109.4

Ultimate Tensile Strength (KSI) Longitudinal 146.1-- 151.6- 156.5- 154.9- 'l'runsverse l47.7 155.2- 157.8

149.2 155.7 Elongation ('7) Longitudinal 5052 48-52 49-51 50-51 Transverse 4550 49-50 50 Hardness (Rockwell) 28 29 28.5 29

6 resistance of the alloys of the present invention to chlo- It may be noted from Table IV that even the smallest ride pitting are set forth in Table III. amount of molybdenum in the alloy of this invention TABLE In significantly increases the resistance of the alloy to sulfuric acid corrosion.

AHOY Number l 2 3 4 5 What IS claimed is Mo Content /2. 0.033 1.02 1.96 2.95 1. An alloy consisting essentially of about 21-45 per- U01 0060 M03 0 cent manganese, about -30 percent chromium,

(grams) about 1-5 percent molybdenum, about 0.85-3 percent The potentiokinetic technique is another method for nitrogen, p to 1 P carbon, P to 2 P Sill measuring the pitting resistance of an alloy to chloride 10 Con and the balance and feslduals Whefem the solutions. In this technique an alloy specimen is placed composltlon 13 Such that! in contact with an appropriate chloride solution and an electrical potential is imposed on the specimen at ing %Mo O'8(%Mn) U'8(%N g creasing voltages until a breakthrough point at which a surge of current passes through the solution. Higher l5 0'5(%Mn)/%Cr+ %Mo+ breakthrough potentials indicate greater resistance to 1) 2 chloride pitting. A significant aspect of the potentiokinetic technique is that the ability of an alloy to selfpassivate may be found by reversing the potential to determine where a high resistance to current flow is obtained after the breakthrough. Alloys with a tendancy to self-passivate display a decrease in current at a voltage near the breakthrough potential. Alloys 2, 3 and 4 all showed better resistance to chloride pitting in potentiokinetic tests than did alloy 1. Of equal significance is that alloys 2, 3 and 4 all showed better selfpassivation properties when the potential on the specimens was reduced than did alloy 1.

The specimens of alloys 1 through 4 inclusive were also subjected to standard corrosion testing to measure their resistance to sulfuric acid. Resistance of an alloy to sulfuric acid is measured by exposing an alloy specimen to sulfuric acid and obtaining the anodic polarization data and the cathodic polarization data and determining their intersection point on a plot of voltage versus current. A correlation is known to exist between the 2. The alloy in accordance with claim 1 comprising from about l-3 percent molybdenum.

3. An alloy in accordance with claim 1 comprising 21-30 percent manganese.

4. An alloy in accordance with claim 1 comprising 15-27 percent chromium.

5. An alloy in accordance with claim 1 comprising 105-1 .5 percent nitrogen.

6. An alloy in accordance with claim 1 comprising 0-0.l5 percent carbon.

7. An alloy in accordance with claim 1 comprising 0-1 .0 percent silicon.

8. A method for producing a substantially nonporous austenitic stainless steel which comprises compositing an alloy containing about 21-45 percent manganese, about 10-30 percent chromium, about l-5 percent molybdenum, about 0.85-3 percent nitrogen, up to 1 percent carbon, up to 2 percent silicon, and the balance iron and residuals, wherein the composition is intersection points of these lines and the resistance of Such that: the specimen to sulfuric acid corrosion. Intersection of 1. %Cr %Mo 0.8(%Mn) l 1.8(%N 0.1) 2 these lines at lower current flows indicates better resis- 28.5 tance to sulfuric acid. Table IV contains data obtained 2. 30(%C %N) 0.5 (%Mn)/%Cr+ %Mo+ in the foregoing test on specimens of alloys 1 through l.5 (%Si) g 1.5

TABLE IV Anodic-cathodic Alloy 1 Alloy 2 Alloy 3 Alloy 4 Polarization 1. ON H2504 intersection a! (milliamp/cm") 4.0 0.002 0.002 0.002

5. ON H2504 intersection at (milliamp/cm 0.00M

4. The resistance to sulfuric acid for all specimens was melting said materials to form a homogeneous liquid measured at 1.0 normal concentration and the resisphase and solidifying the resultant liquid phase without tance of alloy 4 containing nominally 3% molybdenum a dwell period in the temperature range from about was also measured at 5.0 normal sulfuric acid. l,000-l,600F.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,847,599

DATED November 12, 1974 INVENT I Albert G. Hartline III It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Within the abstract, equation 1 contains an improperly typed parenthesis in the term 11.8 (%N 0.1)

Within the abstract equation 2, and in column 1, line 43 and column 6, lines 15 and 40, equation 2 is somewat misleading in that the structure of this fraction would appear to contain only a Mn to Cr ratio. The equation should read:

Column 1, line 48, "At least 10% 2 is required..." I This should read, "At least 10% chromium is required.

Signed and Scaled this fifth Day Of .luly 1977 [SEAL] Arrest:

RUTHC. MASON C. MARSHALL DANN Alteslmg ff Commissioner of Patents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,847,599 DATED November 12, 1974 INVENTOR(S) Albert G. Hartline III It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

1. Within the abstract, equation 1 Contains an improperly typed parenthesis in the term 11. 8(%N 0. l).

2. Within the abstract equation 2, and in column 1, line 43 and column 6, lines 15 and 40, equation 2 is somewhat misleading in that the structure of this fraction would appear to contain only a Mn to Cr ratio. The equation should read:

3. Column 1, line 48, "At least 10% 2 is required This should read, At least 10% chromium is required Signed and sealed this 15th day of July 1975.

(SEAL) Attest C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 

1. AN ALLOY CONSISTING ESSENTIALLY OF ABOUT 21-45 PERCENT MAGANESE, ABOUT 10-30 PERCENT CHROMIUM, ABOUT 1-5 PERCENT MOLYBDEUM ABOUT 0.85-3 PERCENT NITROGEN UP TO 1 PERCENT CARBON UP TO 2 PERCENT SILICON AND THE BALANCE IRON AND RESIDUALS WHEREIN THE COMPOSITION IS SUCH THAT: 1.%CR+%MO+0.8(%MN)-11.8(%N-0.1) 28.5
 2. 30(%C+%N)+0.5(%MN)/%CR+%MO+1.5(%SI) 1.5
 2. 30(%C + %N) + 0.5(%Mn)/%Cr+ %Mo+ 1.5(%Si) > or = 1.5
 2. The alloy in accordance with claim 1 comprising from about 1-3 percent molybdenum.
 2. 30(%C + %N) + 0.5 (%Mn)/%Cr+ %Mo+ 1.5(%Si) > or = 1.5 melting said materials to form a homogeneous liquid phase and solidifying the resultant liquid phase without a dwell period in the temperature range from about 1,000*-1,600*F.
 3. An alloy in accordance with claim 1 comprising 21-30 percent manganese.
 4. An alloy in accordance with claim 1 comprising 15-27 percent chromium.
 5. An alloy in accordance with claim 1 comprising 1.05-1.5 percent nitrogen.
 6. An alloy in accordance with claim 1 comprising 0-0.15 percent carbon.
 7. An alloy in accordance with claim 1 comprising 0-1.0 percent silicon.
 8. A method for producing a substantially non-porous austenitic stainless steel which comprises compositing an alloy containing about 21-45 percent manganese, about 10-30 percent chromium, about 1-5 percent molybdenum, about 0.85-3 percent nitrogen, up to 1 percent carbon, up to 2 percent silicon, and the balance iron and residuals, wherein the composition is such that: 